Often it is useful in an optical system to isolate a single wavelength (or limited range of wavelengths) or to measure the wavelength properties of light impinging on or transmitted through the system. Thin-film interference filters, such as optical edge filters, notch filters, and/or laser line filters (LLF's), can be advantageously used in such systems to isolate a single wavelength or a limited range of wavelengths, or block unwanted light. Many uses for thin-film interference filters are known. For example, U.S. Pat. No. 7,068,430, which is incorporated herein by reference, discusses the use of such filters in fluorescence spectroscopy and other quantification techniques.
In general, thin-film interference filters are wavelength-selective as a result of the interference effects that take place between incident and reflected waves at boundaries between materials having different refractive indices. Interference filters conventionally include a dielectric stack composed of multiple alternating layers of two or more dielectric materials having different refractive indices. Moreover, in a conventional thin-film interference filter, each of the respective layers of the filter stack is very thin, e.g., having an optical thickness (physical thickness times the refractive index of the layer) on the order of a quarter wavelength of light. These layers may be deposited on one or more substrates (e.g., a glass substrate) and in various configurations to provide one or more band-pass, or band-rejection filter characteristics. Further still, in the case of a filter which substantially reflects at least one band of wavelengths and substantially transmits at least a second band of wavelengths immediately adjacent to the first band, such that the filter enables separation of the two bands of wavelengths by redirecting the reflected band, the resulting filter is conventionally called a “dichroic beamsplitter,” or simply a “dichroic” filter.
As used herein, the “blocking” of a filter at a wavelength, or over a range of wavelengths, is typically measured in optical density (“OD” where OD=−log10(T), T being transmission of the filter at a particular wavelength). Conventional filters that achieve high OD values at certain wavelengths or over a range of wavelengths may not necessarily also achieve high transmission (in excess of 50%, for example) at any other wavelengths, or over other ranges of wavelengths. High OD is generally exhibited in a fundamental “stopband” wavelength region, and, as discussed further below, such stopbands have associated with them higher-order harmonic stopband regions occurring at other wavelength regions.
Edge filters, as used herein, are configured so as to exhibit a transmitted spectrum having a defined edge, where unwanted light having wavelengths above or, alternatively, below a chosen “transition” wavelength λT is blocked, and where desired light is transmitted on the opposite side of λT. In addition, as used herein, edge filters are configured such that the wavelength range over which the transmission characteristics transition from “blocking” to “transmitting” or vice versa is substantially smaller (i.e., at least one order of magnitude smaller, and usually two orders of magnitude smaller or more) than the wavelength ranges over which the “blocking” and “transmitting” characteristics are exhibited. Edge filters which transmit optical wavelengths longer than λT are called long-wave pass (LWP) filters, and those that transmit wavelengths shorter than λT are short-wave pass (SWP) filters.
U.S. patent application Ser. No. 12/129,534, the contents of which are incorporated by reference, includes a discussion of the spectral transmission characteristics of idealized LWP and SWP filters, respectively. For example, an idealized LWP filter completely blocks light with wavelengths below λT, and completely transmits wavelengths above λT. Conversely, an idealized SWP filter completely transmits light with wavelengths below λT, and completely blocks light with wavelengths above λT.
Edge steepness and the relative amount of transmitted light can be important parameters in many filter applications. As discussed above, an idealized edge filter may be considered to have a precise wavelength edge that is represented by a vertical line in a plot of transmission versus wavelength at wavelength λT. As such, an idealized filter has an “edge steepness” (i.e. the change in wavelength over which the transmission characteristics transition from a “blocking” to “transmitting” or vice versa) of 0 at λT. However, real edge filters change from blocking to transmitting over a small but non-zero range of wavelengths, with increasing values of edge steepness reflecting an edge that has increasingly less slope. The transition of a real edge filter is therefore more accurately represented by a non-vertical but highly sloped line at or near λT. Similarly, while an ideal edge filter may be considered to transmit all of the incident light in the transmission region (transmission T=1), real edge filters have some amount of transmission loss, invariably blocking a small portion of the light in the intended transmission region (T<1). As a result, the edge steepness of a real edge filter can be considered to be dependent upon the transmission range over which it is defined.
Systems that benefit from wavelength selection through optical filtering can be expected to benefit from an optical filter that exhibits high transmission in the filter passband regions, highly sloped spectral edges, and high out-of-band blocking of undesired wavelengths. Consequently, thin-film optical filters, which are capable of providing close to 100% transmission, exhibit highly sloped spectral edges (transitioning from high transmission to blocking of optical density 6 or higher in less than 1% of the edge wavelength), and exhibit blocking of optical density 6 or higher over wide spectral regions, can be considered beneficial to such systems.
However, thin-film filters are also conventionally regarded as “fixed” filters, in that each thin-film filter has a certain spectral function that is conventionally usable over a limited wavelength region. To the extent a conventional system utilizing a first thin-film filter with a given spectral function may require a different spectral function, such a system would require a means for swapping the first thin-film filter with a different one exhibiting the required spectral function. Mechanical means to perform filter swapping are known, such as the use of filter wheels, but these are conventionally large, relatively slow (i.e., minimum switching times are conventionally 50 to 100 ms), and permit only a limited number of filters (e.g., conventional wheels contain from 4 to 12 slots for such fixed filters). Thus, conventional filter systems that can benefit from the use of thin-film filters and also benefit from the availability of variable, or dynamic, spectral characteristics will have limited application as a result of the size, speed, and filtering function flexibility characteristics discussed above.
Conventionally, when it is desired to change a spectral function associated with a wavelength, or wavelength range, to a different wavelength or wavelength range at a relatively high speed, diffraction gratings have been used. Diffraction gratings permit any wavelength within a range to be selected, and can accommodate a change of wavelength relatively quickly since only rotation of the grating is required. Thus, gratings are the basis of most scanning spectral measurement systems. However, gratings do not offer very good spectral discrimination. For example, spectral edges are conventionally not highly sloped and out-of-band blocking is conventionally poor. Moreover, concatenating multiple diffraction grating spectrometers together in order to address some of these deficiencies tends not to completely alleviate the poor out-of-band blocking, reduces the overall transmission, and further makes the instrument very large. Grating-based systems also conventionally exhibit limited control over the filter bandwidth. That is, while the bandwidth may be adjusted (usually by adjusting the width of a mechanical slit), the slope of the edge will decrease as the bandwidth increases. Further still, at least one spatial dimension is conventionally required for spreading out light rays in a wavelength-dependent manner for grating-based systems, and thus it is not possible to measure a two-dimensional imaging beam directly (at one instant of time) with a conventional diffraction grating. For example, so-called “imaging grating spectrometers” provide one-dimensional spatial imaging with the second dimension reserved for spectral information. To the extent a second spatial dimension is present in the imaging beam, and to the extent it is static, it may be captured separately as a function of the time it takes the diffraction grating to sample the second available dimension.
Other conventional filter systems that are designed to exhibit variable spectral functionality include angle-tuned thin-film filters, liquid crystal tunable filters, acousto-optic tunable filters and linear-variable tunable filters. Each is discussed below.
With regard to angle-tuned thin-film filters, it is known that the spectrum of a thin-film filter shifts toward shorter wavelengths when the angle of incidence (AOI) of light upon the filter is increased from 0 degrees (normal incidence) to larger angles (see Optical Waves in Layered Media, Pochi Yeh, Wiley, New York, 1988, Section 7.6). This shift is generally described by the equation
                              λ          ⁡                      (            θ            )                          =                              λ            ⁡                          (              0              )                                ⁢                                    1              -                                                                    sin                    2                                    ⁡                                      (                    θ                    )                                                                    n                  eff                  2                                                                                        (        1        )            where θ is the AOI and neff is the “effective index of refraction,” which is generally dependent upon the filter design and the two orthogonal states of polarization (i.e., “s” and “p”). This effect may be used to tune the spectrum of an optical filter over a limited range of wavelengths. Conventionally, however, the filter spectrum becomes distorted at larger angles, and the wavelength shift can be significantly different for s- and p-polarized light, leading to a strong polarization dependence at higher angles.
As an example, FIG. 1 depicts calculated spectra of an exemplary multi-cavity Fabry-Perot thin-film filter configured to provide a narrow passband (about 2 nm) at 561 nm. Curves 101 and 102 are calculated p- and s-polarized transmission spectra associated with an AOI of 60 degrees, curves 111 and 112 are calculated p- and s-polarized transmission spectra associated with an AOI of 45 degrees, curves 121 and 122 are calculated p- and s-polarized transmission spectra associated with an AOI of 30 degrees, and curves 130 are calculated p- and s-polarized transmission spectra associated with an AOI of 0 degrees. As exhibited in FIG. 1, the passband becomes narrower for s-polarization and wider for p-polarization as the AOI increases from 0 to 60 degrees. In addition, the peak in the transmission curve exhibits more of a wavelength shift as a function of AOI for p- than for s-polarized transmitted light. As a result, at higher AOI values, a conventional thin-film filter exhibits strong polarization dependence. By way of example only, the conventionally usable range of wavelengths over which this exemplary filter may be tuned away from 0 degrees AOI at 561 nm (that is, before excessive distortion of the spectrum occurs) is approximately 10 to 15 nm of wavelength, or about 2 to 3% of the starting wavelength.
Conventional angle-tuned thin-film filters consistent with the exemplary thin-film filter of FIG. 1 are used in a number of commercial devices and instruments for fiber-optic telecommunications test and measurement and system applications which are configured to operate near 1550 nm. Conventional tuning ranges for these exemplary devices are about 30 to 40 nm near 1550 nm, which corresponds to about 2 to 2.5% of the starting wavelength. U.S. Pat. Nos. 5,481,402, 5,781,332, and 5,781,341 also disclose angle-tunable thin-film filters for fiber-optic/telecom applications, including the use of a multi-cavity Fabry-Perot thin-film bandpass filter with a passband near 1550 nm positioned in the vicinity of collimated light coupled to an optical fiber, and rotated to achieve angle tuning properties.
For applications that do not rely upon precise laser lines or fiber-optic telecommunications, wider passbands are typically desired. By way of example only, FIG. 2 depicts calculated spectra associated with an exemplary multi-cavity Fabry-Perot thin-film filter configured to exhibit a passband of approximately 20 nm at full-width-half-maximum (FWHM). Curves 201 and 202 are calculated p- and s-polarized transmission spectra associated with an AOI of 60 degrees, curves 211 and 212 are calculated p- and s-polarized transmission spectra associated with an AOI of 45 degrees, curves 221 and 222 are calculated p- and s-polarized transmission spectra associated with an AOI of 30 degrees, and curves 230 are calculated p- and s-polarized transmission spectra associated with an AOI of 0 degrees. The behavior exhibited by the exemplary thin-film filter of FIG. 2 is analogous to that of the filter of FIG. 1. For example, the passband wavelengths shift to shorter wavelengths at higher AOI. Moreover, the transmission characteristics over the passband region become increasingly distorted (i.e., exhibit a larger range of variability) at higher AOI. Further, the passband region (as defined by the stopband edges) becomes narrower for s-polarization and wider for p-polarization as the AOI increases. Further still, the passband exhibits more of a wavelength shift as a function of AOI for p- than for s-polarized light. All of these characteristics conventionally result in an increasingly distorted spectrum at higher AOI, thus reducing the useful tuning range of such a conventional filter.
Many fluorescence imaging and quantization applications can benefit from filters with passbands that are wider than even the exemplary thin-film filter of FIG. 2. For example, such applications can benefit from passbands that are 30 to 50 nm wide or more (at visible wavelengths). For filters with such wide passbands, and especially for applications involving incoherent light, a conventional filter may be formed from a combination of long-wave pass and short-wave pass edge filters (such as those disclosed in U.S. Pat. Nos. 6,809,859 and 7,411,679 B2). Such edge filters are formed from the edge of a “stopband” spectral region that results from an approximately quarter-wave stack of high- and low-index thin-film layers. One limitation with such a combination of edge filters, however, is that conventional edge filters exhibit polarization splitting when operated at a non-zero angle of incidence.
By way of example only, FIG. 3 depicts calculated spectra associated with a combination of long-wave-pass and short-wave-pass filter coatings (FWHM˜35 nm). Curves 301 and 302 (the latter of which is essentially flush with the abscissa) are calculated p- and s-polarized transmission spectra associated with an AOI of 60 degrees, curves 311 and 312 are calculated p- and s-polarized transmission spectra associated with an AOI of 45 degrees, curves 321 and 322 are calculated p- and s-polarized transmission spectra associated with an AOI of 30 degrees, and curves 330 are calculated p- and s-polarized transmission spectra associated with an AOI of 0 degrees. The exemplary combination of edge filters associated with FIG. 3 exhibits qualitatively similar behavior to the exemplary multi-cavity Fabry-Perot type filters of FIG. 2, but there is significantly more distortion of the spectrum as the AOI is increased from normal incidence. In particular, the passband ripple can render the filter unusable for p-polarization light, and the passband for s-polarization diminishes to where it is essentially eliminated for light at an AOI of 60 degrees. Consequently, the useful angular tuning range of the exemplary combination of edge filters associated with FIG. 3 is only about 10 degrees to 15 degrees for this filter, resulting in a wavelength tuning range of about 0.5%-1%.
More generally, increasing the AOI of a conventional interference filter from normal generally affects the spectrum of the filter in two ways. First, the features of the filter spectrum are shifted to shorter wavelengths. Second, as the angle of the filter is further increased from normal, the composite spectra of the filter progressively exhibits two distinct spectra: one for s-polarized light and one for p-polarized light. As used herein, the relative difference between the s- and p-polarized spectra at a given point is generally called “polarization splitting.”
To illustrate this effect, reference is made to FIGS. 4A and 4B which are plots of polarization splitting as a function of AOI for a quarter wave stack based on SiO2 and Ta2O5 centered at 500 nm. In the plot of FIG. 4A, the bandwidths of the stopbands associated with light of s-polarization and p-polarization are shown, with the bandwidths measured in so-called “g-space.” The parameter g=λ0/λ is inversely proportional to wavelength and therefore directly proportional to optical frequency, and equals 1 at the wavelength λ0 which is at the center of a fundamental stopband associated with a stack of thin film layers each equal to λ0/4n in thickness, where n is the index of refraction of each layer. The bandwidth in g-space is therefore equal to the difference between λ0/λS and λ0/λL, where λS and λL are the short-wavelength and long-wavelength edges of the stopband, respectively. The polarization splitting in g-space is thus simply one half of the difference between the bandwidths in g-space for s-polarized and p-polarized light. As shown in FIG. 4B, the stack exhibits polarization splitting of about 0.04 g-numbers when operated at 45 degrees AOI. Increasing AOI to 60 degrees results in polarization splitting of almost 0.07 g-numbers. Decreasing AOI to 20 degrees results in polarization splitting of less than 0.01 g-numbers.
By way of example only, conventional dichroic optical filters exhibit substantial polarization splitting, particularly when operated at about 45 degrees AOI. This polarization splitting arises from the particular construction of a dichroic filter. As mentioned previously, conventional dichroic filters are generally made up of alternating thin material layers having differing refractive index. In addition to the refractive index of each layer being different than that of an adjacent layer, the effective refractive indices of each individual layer differ with respect to different polarizations of light. That is, the effective refractive index for a layer is different for p-polarized light than it is for s-polarized light. As a result, s-polarized and p-polarized light are shifted to different degrees upon passing through each layer in a dichroic filter. This difference in shift ultimately offsets the filter spectra corresponding to these differing polarizations, resulting in polarization splitting.
If a conventional dichroic filter is based on the first order stopband of an angle-matched quarter-wave stack, estimating the polarization splitting between the stopband bandwidths of the filter is relatively straightforward. That is, assuming the dichroic filter is made up of two materials having indices of nH, and nL, respectively, at 45 degree angle of incidence, the effective indices can be calculated as follows:
                              n          L          S                =                                            n              L              2                        -                                          sin                2                            ⁡                              (                AOI                )                                                                        (        2        )                                          n          H          S                =                                            n              H              2                        -                                          sin                2                            ⁡                              (                AOI                )                                                                        (        3        )                                          n          L          P                =                              n            L            2                                                              n                L                2                            -                                                sin                  2                                ⁡                                  (                  AOI                  )                                                                                        (        4        )                                          n          H          P                =                              n            H            2                                                              n                H                2                            -                                                sin                  2                                ⁡                                  (                  AOI                  )                                                                                        (        5        )            where:                AOI is the incident angle in air, which is assumed to be the incident medium;        nLP and nLS are the effective refractive index of the low index material in the dichroic stack for p-polarized light and s-polarized light, respectively;        nHP and nHS are the effective refractive index of the high index material in the dichroic stack for p-polarized light and s-polarized light, respectively; and        nH2 and nL2 are the squares of the high and low refractive indexes, respectively, associated with the two materials, and which are independent of polarization.        
The bandwidths and polarization splitting of the first-order stopband for the two polarizations may then be calculated as follows:
                              Δ          ⁢                                          ⁢                      g            S                          =                              4            π                    ⁢                                    sin                              -                1                                      (                                                            n                  H                  S                                -                                  n                  L                  S                                                                              n                  H                  S                                +                                  n                  L                  S                                                      )                                              (        6        )                                          Δ          ⁢                                          ⁢                      g            P                          =                              4            π                    ⁢                                    sin                              -                1                                      (                                                            n                  H                  P                                -                                  n                  L                  P                                                                              n                  H                  P                                +                                  n                  L                  P                                                      )                                              (        7        )                                          PS          g                =                                            Δ              ⁢                                                          ⁢                              g                S                                      -                          Δ              ⁢                                                          ⁢                              g                P                                              2                                    (        8        )            where:                ΔgS and ΔgP are the bandwidths of the first order (fundamental) stopband for s-polarized light and p-polarized light, respectively, in g-space; and        PSg is the polarization splitting for the first-order stopband in g-space.Alternatively, the polarization splitting may be expressed in terms of wavelength. For example,        
                              PS          λ                =                                            λ              0                                      1              -                              Δ                ⁢                                                                  ⁢                                                      g                    S                                    /                  2                                                              -                                    λ              0                                      1              -                              Δ                ⁢                                                                  ⁢                                                      g                    P                                    /                  2                                                                                        (        9        )            where:                PSλ is the polarization splitting of the long-wavelength edge of the fundamental stopband (the edge associated with a long-pass filter).Often this value is expressed as a dimensionless value by taking its ratio to the average wavelength of the edges associated with s- and p-polarizations and expressing it as a percentage.        
Polarization splitting has been utilized to design polarizing filters where high transmission and blocking are achieved for p- and s-polarizations, respectively, over a defined wavelength band. However, in the context of edge filters and beamsplitter optical filters, polarization splitting severely limits the edge steepness of light having average polarization. Systems disclosed as utilizing such combinations of edge filters are those described in U.S. Pat. Nos. 3,864,037, 5,591,981, and 5,852,498.
U.S. Pat. Nos. 4,373,782 and 5,926,317 disclose an approach to minimizing the conventional spectral distortion of combinations of angle-tuned thin-film edge filters. For example, U.S. Pat. Nos. 4,373,782 and 5,926,317 disclose the use of a multi-layered multi-cavity Fabry-Perot type bandpass filters to allow some control of the spectral positioning of the passbands for s- and for p-polarization at a particular non-zero AOI. At a particular AOI (e.g., 45 degrees), it is possible to position the narrower s-polarization passband such that one of its edges lines up with the corresponding edge of the wider p-polarization passband, thus approximately eliminating the polarization splitting associated with either the long- or the short-wave-pass edge. Because of the significant difference in passband bandwidths for the two polarizations, however, and the correlation between passband bandwidth and edge steepness in a multi-cavity Fabry-Perot type filter, the two edges will conventionally have significantly different edge steepness. Furthermore, the passband width is conventionally very limited for edge filter applications due to the formation of the Fabry-Perot passband within the stopband region.
In a further method proposed by Thelen to minimize polarization splitting (see A. Thelen, Design of Optical Interference Coatings, McGraw Hill, 1989), the method utilizes tuning spacers of a multi-cavity Fabry-Perot bandpass filter to align the edges of spectrum of s- and p-polarized light. However, this method has significant limitations when used to create dichroic filters. Specifically, in this method, the starting layer structure is that of a multi-cavity Fabry-Perot bandpass filter with spacer layers having optical thickness equal to multiple half-waves of the reference wavelength used to define the associated stopband. In addition, the edge of the resulting dichroic must be essentially at the center of the associated stopband. It has been shown that decreasing stopband bandwidth can result in a corresponding decrease in polarization splitting. In the case of a filter having a second order stopband, the bandwidth of the stopband is proportional to the material mismatch in the dielectric stack making up the filter, where “mismatch” refers to the deviation of the layer thicknesses from one quarter of a wavelength, while keeping the sum of the thicknesses of each pair of high- and low-index layers equal to approximately one half of a wavelength. The greater the mismatch, the higher the degree of polarization splitting, and vice versa. Thus, it has been shown that polarization splitting can be minimized by utilizing different (e.g., higher-order) stopbands and adjusting material mismatch in the dielectric stack making up a dichroic filter.
However, while this method is effective, small mismatch always results in a filter having a narrow blocking region and lower blocking level, which is often not acceptable. Enhancement of the blocking region can be achieved, but only by increasing the number of layers in the dielectric stack. As a result, the performance of a traditional dichroic filter based on a second order stopband is typically limited by the maximum coating thickness allowed by the manufacturing process.
Higher-order stopbands are one reason why it is difficult to achieve high transmission at wavelengths shorter than those over which high blocking occurs. A stopband is a range of wavelengths over which transmitted light is strongly attenuated (T≦10%) due to constructive interference of the many partial waves of light reflected off of a structure with a periodic or nearly periodic variation of the index of refraction, as found in a thin-film interference filter. For a “quarter wavelength stack” structure comprised of alternating layers of high- and low-index materials, each of which is approximately one quarter of a particular wavelength λ0 thick (in the material), the “fundamental” stopband is roughly centered on λ0 and ranges from approximately λ0/(1+x) to λ0/(1−x), where x is related to the high and low index of refraction values, nH and nL, respectively, according to
                    x        =                              2            π                    ⁢                      arcsin            ⁡                          (                                                                    n                    H                                    -                                      n                    L                                                                                        n                    H                                    +                                      n                    L                                                              )                                                          (        10        )            
If the layer-to-layer index of refraction variation is not a purely sinusoidal variation, but rather changes abruptly, as is typically the case in a multi-layer thin-film interference filter, higher-order stopbands exist at shorter wavelengths. For example, a quarter-wave stack having such abrupt refractive index changes exhibits “odd-harmonic” stopbands that occur approximately at the wavelengths λ0/3, λ0/5, etc., and that range from approximately λ0/(3+x) to λ0/(3−x), for the third-order stopband, λ0/(5+x) to λ0/(5−x) for the fifth-order stopband, and so on. If the layers are not exactly a quarter-wave thick, there may also be “even-harmonic” stopbands that occur approximately at the wavelengths λ0/2, λ0/4, etc.
In general, known filters achieve high blocking over a wide range by utilizing a fundamental stopband, by combining multiple fundamental stopbands, or by “chirping” (gradually varying) the layers associated with one or more fundamental stopbands. Regardless of the approach, the higher-order harmonic stopbands associated with these blocking layers inhibit transmission at wavelengths shorter than the fundamental stopband or stopbands.
Liquid-crystal tunable filters (LCTF) are also designed to exhibit variable spectral functionality. LCTF's are based on a Lyot-Ohman polarization interference filter configuration (Optical Waves in Layered Media, Pochi Yeh, Wiley, New York, 1988, Section 10.1), which is a series of birefringent crystal plates separated by linear polarizers. Each “stage” of this filter produces a sinusoidal variation of transmission as a function of wavelength, and the combination of many stages each producing a sinusoidal transmission curve with a different frequency results in constructive interference of the overall transmission characteristic at just one wavelength: the filter passband. An LCTF adds birefringent liquid-crystal plates to the fixed birefringent plates in each section, such that an adjustment of the liquid-crystal birefringence by application of a voltage across these plates modifies the wavelength at which the constructive interference occurs, thus tuning the wavelength of the passband.
LCTF filters can be designed to tune over many 100's of nm. In particular, filters are available for tuning over the entire visible wavelength range (<400 nm to >700 nm). Like angle-tuned thin-film filters, they are capable of large apertures and two-dimensional imaging. Significant shortcomings of these filters include poor transmission, poor edge steepness and out-of-band blocking, fixed bandwidth, and low laser damage threshold (LDT). And, because they are based on polarization, they are strongly polarization dependent. In fact, they transmit only linearly polarized light, so the maximum transmission of unpolarized light is 50%, and in fact the transmission of even the polarized light passed through the filter tends to be about 50% (which results in 25% overall transmission for unpolarized light). An example of a filter based on ferroelectric liquid crystals is disclosed in U.S. Pat. No. 5,132,826.
A further conventional filter system designed to exhibit variable spectral functionality includes acousto-optic tunable filters (AOTF). The AOTF is based on the diffraction of light off of a volume grating formed by acoustic shear waves traveling in a single crystal of a material like tellurium dioxide (TeO2). The waves are produced by a radio frequency (RF) transducer bonded to one side of the crystal. Light transmitted through the crystal is separated into three waves, each traveling in a different (angular) direction: a zeroth order undiffracted beam; a diffracted beam of one orientation of (linear) polarization; and a diffracted beam of the other (orthogonal) polarization. The undiffracted beam contains all wavelengths, whereas the diffracted beams contain only wavelengths within a narrow passband due to diffraction off of the volume hologram. To use the device as a filter, usually the undiffracted beam and one of the polarized diffracted beams are blocked, and the second polarized diffracted beam is used as the transmitted light.
AOTF filters also can exhibit a wide tuning range (like the LCTF it is also capable of tuning over 100's of nm), and can exhibit a high tuning speed (wavelength switching times as fast as 10 microseconds are possible, compared to milliseconds and above switching times for all other technologies). The most significant disadvantages include poor edge steepness and out-of-band blocking, lack of adjustable bandwidth, and very small apertures (typically 3 to 10 mm at most), which limits the usefulness for imaging applications. And, like the LCTF, AOTF's only use linearly polarized light, and thus for unpolarized light applications at least 50% of the light is lost. U.S. Pat. No. 5,796,512 provides an example of an imaging system that uses an AOTF.
A further conventional filter system designed to exhibit variable spectral functionality includes linear-variable tunable filters (LVTF): Linear (as well as circular) variable thin-film filters are based on the concept of non-uniform thin-film layer thickness variation as a function of position along a linear direction (for linear-variable filters) or around the azimuthal direction of a round filter (for circular-variable filters). As a result, the spectral properties of the LVTF filter, which scale with the layer thickness, vary spatially as well. Thus, by varying the location of an optical beam on the filter—either by moving the beam across the filter or moving the filter across the path of the beam—the spectral properties (such as the edge position of an edge filter or the passband wavelength of a bandpass filter) can be varied. U.S. Pat. No. 6,700,690 describes a combination of two LVTF's—one a long-wave-pass filter and the other a short-wave-pass filter—such that when they are translated independently not only the center wavelength formed by the combination of the two filters but also the bandwidth can be adjusted.
LVTF's share some of the characteristics of other tunable thin-film filters, including transmission properties, imaging properties, and they exhibit a relatively high laser damage threshold. In addition, because light is always incident at or near 0 degrees AOI they can be polarization insensitive. Moreover, using the configuration disclosed in U.S. Pat. No. 6,700,690, the bandwidth can be adjusted arbitrarily for any center wavelength. Disadvantages include poorer spectral performance (edge steepness) due to the variation of the spectral properties across a non-zero width optical beam, and slow tuning speed due to the need to translate the filters mechanically.
There exist a number of other thin-film and non-thin-film approaches to tunable optical filters capable of selecting a band of wavelengths from a two-dimensional imaging beam, including: bulk Sagnac, Mach-Zehnder, and Michelson type interferometers, Fabry-Perot interferometers, and thin-film filters with actively adjustable layers and/or substrate or tuned by mechanically induced changes in the optical properties via the stress-optic effect. As an example, “Tunable thin-film filters: review and perspectives,” M. Lequime, Proceedings of the SPIE, Vol. 5250, pp. 302-311, 2003 (C. Amra, N. Kaiser, H. A. Macleod, Eds.) describes several such filter concepts.
Accordingly, there is a need to provide an optical filter with the spectral and two-dimensional imaging performance characteristics of thin-film filters and the center wavelength tuning flexibility of a diffraction grating.